Tech Briefs

Modified Coaxial Probe Feeds for Layered Antennas

Coaxial shields are connected to radiator and ground planes at standing-wave nodes.

In a modified configuration of a coaxial probe feed for a layered printed-circuit antenna (e.g., a microstrip antenna), the outer conductor of the coaxial cable extends through the thickness of at least one dielectric layer and is connected to both the ground-plane conductor and a radiator-plane conductor. This modified configuration simplifies the incorporation of such radio-frequency integrated circuits as power dividers, filters, and low-noise amplifiers. It also simplifies the design and fabrication of stacked antennas with aperture feeds.

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In the Modified Coaxial Probe Feed, the outer conductor of the coaxial cable constitutes a short circuit between the ground plane and the radiator patch.
It is often desirable to incorporate the aforementioned integrated circuits into antenna structures in order to obtain better performance or more compact packaging than would be achievable by packaging these circuits as electrically connected but structurally separate units. In a typical conventional coaxial probe feed configuration, the integrated circuitry is located beneath the ground plane. Incorporation of the integrated circuitry into the antenna entails difficulty in (1) making solder connections at locations that are partly or totally inaccessible and (2) ensuring the necessary precise alignments between hidden coupled transmission lines. In contrast, in the modified configuration, the integrated circuitry is mounted on the outside, where it is visible and accessible.

The figure shows examples of simple conventional and modified coaxial probe feeds. In the example of the conventional configuration, the outer conductor of a coaxial cable is connected to, and terminated at, a ground-plane metal layer, while the central conductor extends through a single dielectric layer to a connection with a patterned metal radiator element (e.g., a microstrip patch).

In the example of the modified configuration, the outer conductor of the coaxial cable extends through the thickness of the dielectric layer between, and is electrically connected to, both the ground-plane metal conductor and the patterned metal layer on the radiator plane. The central conductor of the coaxial cable extends through the thickness of the dielectric substrate of an integrated circuit to the integrated circuit, which is located on the outer surface. The integrated circuit then excites the cavity formed by the bounding top and bottom planes, by means of an electrical connection passing through an aperture in the patterned metal radiator element (see figure).

Although the coaxial outer conductor constitutes a short circuit between the ground and radiator planes, the effect of the short circuit is minimal because care is taken to locate the coaxial intrusion at a node of the standing-wave mode of the antenna electro magnetic field; that is, the effect of the short circuit is minimal because at its chosen location, the electric field is nominally zero in the absence of perturbations. In the ideal case, the diameter of the outer conductor of the coaxial cable would be zero and there would be no perturbations. In reality, the outer conductor has a finite diameter, leading to a slight shift of the resonance frequency of the antenna. However, the resonance frequency is easily adjusted by slightly changing the length of the antenna.

In some designs, the metal radiator and ground plane are intentionally short-circuited by use of a post at a specified location to shift the resonance frequency by a specified amount. In an application of the modified configuration to such a design, the coaxial intrusion could be substituted for the post.

The modified feed can also be applied to the so-called PIFA (planar inverted-F antenna), which has achieved great popularity due to its compact size. In this application, the coaxial intrusion provides all or part of the required short circuit between the ground plane and the patterned metal layer on the radiator plane.

This work was done by Patrick W. Fink, Andrew W. Chu, Justin A. Dobbins, and Greg Y. Lin of Johnson Space Center.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Johnson Space Center, (281) 483-0837. Refer to MSC-23549.